A surface acoustic wave (SAW) is an acoustic wave which travels along the surface of a material having a certain elasticity, with an amplitude that decreases exponentially with the depth of the substrate. SAW devices which make use of such waves are very commonly used as filters, but are also used as sensors. SAW devices are noted for great sensitivity, low energy consumption and the possibility of being operated within the range of radio frequencies, which is advantageous for the wireless transmission of the data.
It is known to use SAW sensors in tire-wheel elements, as described for example in the document WO 2006/058918 where a tire temperature is measured by a sensor of that type. In such applications, in particular SAW sensors of the “differential resonator” type can be used, which comprise two resonators with different resonance frequencies (f1 and f2) and are dimensioned so that the difference between the frequencies (f2−f1) is a function of the temperature.
One of the difficulties connected with the use of SAW sensors is the poor reproducibility between different sensors, illustrated in FIG. 1 for sensors of the “differential resonator” type. In this particular case, the value of the temperature can only be reproduced with a precision of around 20° C., which is insufficient for most applications at least in the context of tires.
Several solutions for this problem have been proposed. In particular, each SAW sensor can be associated with a Radio Frequency Identification (RFID) device and the calibration function can be stored on the RFID device (see for example the U.S. Pat. No. 7,129,828). When the SAW sensor is interrogated, the reader also interrogates the RFID device to retrieve the corresponding calibration function. This approach indeed allows to obtain satisfactory calibration, but it has a number of disadvantages. Besides high cost, the need to have two emitters can be mentioned (for example, of 433 MHz for the SAW sensor and 915 MHz for the RFID device), as well as two antennas, etc.
Another approach has been proposed in the document WO 2005/087518. This describes the use of a memory card associated with each sensor and containing the calibration parameters of the sensor. The memory cards are inserted into the reader once the sensors have been installed on the vehicle. This achieves good calibration, but such a system is prone to the risk of poor correspondence, i.e. the association of a sensor with a memory card containing the calibration function of another sensor.
Finally, mention should be made of the work carried out by manufacturers of SAW devices to improve the reproducibility of SAW sensors, in particular by collective fabrication processes. Very considerable advances have been achieved recently, but it is likely that the new production processes will result in cost increases.
In summary, the current tendency is to solve the problems of SAW sensors calibration by means of more complex devices and/or production processes, and this entails higher cost.